Smuxi already supports spell checking on Linux by using the libgtkspell library. This library needs to be ported/build on Windows so Smuxi can also use it there. Or Smuxi needs to use a different library that Windows supports.

05:45:23 <Codrus> Is it planned to ever have spellcheck for the windows version?
05:54:50 <pereba> they could implement Windows 8+ native spell check, Hexchat integrated that recently in dev version.